In many states it is even illegal to buy/sell a kitten that is less than 8 weeks. 8 weeks is typically when the mother weans, so people tend to get rid of them "as soon as possible", which means 8-9 weeks, but Batfink is right, optimally they will stay with the mother for 12 weeks because it is in the 10-12 week range that they learn really crucial social skills from their family, including mother and siblings.